Extracerebral bony landmarks which are presently used in experimental animals are one of the main sources of errors finally resulting in stereotaxic misplacements. The objective of the present project is to construct a stereotaxic atlas of the Rhesus monkey brain using an intracerebral coordinate system in order to increase precision of surgical procedures in this species. X-ray ventriculography will be used for visualization of intracerebral references (foramen Monro, anterior and posterior commissures, cerebral aqueduct and III and IV ventricles) during surgery. The CA-CP (inter-commissural) line and the midline will be used as the main landmarks on ventricular radiographs and a stereotaxic brain atlas will be constructed within the same coordinate system. The atlas can be used in all neuroanatomical, neurophysiological and behavioral experiments where precise placements in minute brain structures are required. The advantage of this atlas is that it can be used also for non-invasive stereotaxy; for example, computerized tomography could be substituted for ventriculography in the future.